The preferred power-conversion method for modern electronic equipment is the Switching Mode Power Supply (SMPS). It is characterized by high efficiency and resulting low wasted heat dissipation.
In order to create the SMPS circuit, various inductive components are required. As modern electronic devices shrink in dimensions, correspondingly all inductive components must be reduced in size. One method for achieving this reduction in size is to use so-called planar inductive structures, where the magnetic-path components (core) are snapped around flat windings incorporated into the printed-circuit board (PCB). A PCB may actually be carrying all components of the device (main PCB). More commonly, however, a PCB carrying an inductive component may be a separate “winding” PCB attached to the main PCB, while utilizing the same or different number of copper layers as in the main PCB.
Effective mechanical attachment and low-resistance electrical termination (connections) of the windings to the main PCB is difficult, and consumes significant areas on both the main PCB and the winding PCB. In addition, typical termination strategies make use of soldered connections, leading to a fundamental problem when such strategies are applied to joining planar components. When two pieces of metal are joined by soldering, there could be two basic situations, as shown in FIG. 1: (a) Two metal pieces are butted together and affixed with a single-meniscus of solder at the joint; or (b) Two pieces of metal spaced apart, but joined by a double-meniscus of solder between the two pieces of metal. In both cases, there is at least one area where solder forms meniscus (due to surface tension when the solder is in liquid form). If solder does not form a concave meniscus—the solder joint is not going to be strong, and the integrity of the joint will not be reliable.
Ideally, the windings of a transformer occupy the whole available “window” of the magnetic core. It is necessary for two reasons: (1) To be able to fit as much copper as possible, so that the DC resistance (and thus the heat losses) is as low as possible, and (2) to prevent possible formation of plasma due to empty pockets of air.
The planar transformer is typically constructed with winding made with exactly the same technology as the ordinary PCBs—with copper conductors (formed by etching or electro deposition) on the top, bottom, and possibly several internal layers. More than a single PCB may form all the windings of a single transformer—thus there is a need to be able to electrically connect these windings between each other, and to the “main” PCB.
One proposed solution in the art (see PCT Publication WO 01/37624 A1) specifically require a certain distance “k” between the conductors on two PCBs to be joined—in order to allow sufficient space for the solder to penetrate and to form “double meniscus”, as shown in FIG. 1. However, this proposed solution does not meet the need of having PCBs containing planar inductive components located effectively without any space between the PCBs.
Another proposed solution to terminating stacks of PCBs together in the construction for a “surface mountable” planar transformer is described in U.S. Pat. No. 6,073,339 ('339 patent). In the '339 patent, termination of the complete stack of PCBs that constitute the transformer's windings is achieved similarly to the well-known method of mounting SMT (Surface Mount Technology) components. The solder joint between the “windings” PCBs and the “main” PCB is similar to illustration of the “butted” joint described in FIG. 1. While the '339 patent does not specifically mention the fact that the “main” PCB also carries parts of the winding and is located inside of the magnetic path of the core, it suggests that the “main” PCB has a cut-out that matches the size of the magnetic core. A closer look at the solder termination used in the '339 patent reveals that this method cannot achieve zero space between PCBs. There would be significant space between the PCBs being joined, as shown in the depiction of the solder joint shown in FIG. 2, even if the metal traces touch directly (which is not the case during typical SMT assembly operations, as the top PCB “floats” on the molten solder). The area of the solder termination shown in FIG. 2 is located outside the thickness of the PCB, which necessarily causes an undesirable air space.
Another example of a proposed solution in the art (U.S. Patent Application Publication 2006/0279394 ('394 publication)) discloses a method for connecting several PCBs (that are utilized as windings in the planar transformer) to terminals that would in turn be connected to another “main” PCB or directly to wires. It can be clearly seen that the '394 publication does not provide a method that allows PCBs to be touching, as the solder meniscus has to be formed on both top and bottom of each PCB that is soldered to the common terminal. In the '394 publication, the interloper terminals are absolutely necessary, as they provide the electrical connection path from the “winding” PCBs to “main” PCB or to wires.
The above-mentioned examples of existing art leave a need to more-effectively make these PCB terminations to enable planar transformers.